White films are, compared to papers made of pulp, as synthetic papers more excellent in water resistance, strength, smoothness, etc., used as general packaging materials, display items such as labels, posters, stickers, thermal transfer recording receiving sheets for thermal transfer recording system, reflectors or reflection plates or the like.
Among them, the thermal transfer recording system is a printing system in which an ink ribbon having an ink layer which is a dye containing layer and a receiving sheet are superposed and by heating from the ink ribbon side with a thermal head, the color material or the color material component is melted or sublimated to be transferred and printed in fine dots (half-tone) onto the receiving sheet.
In recent years, when the receiving sheet for thermal transfer recording is printed, heat load received by the receiving sheet becomes high due to increasing printing speed. And, since process conditions have become severe and due to down-sizing of printing machines, the environment in which the receiving sheet for thermal transfer recording is used is becoming severe year by year. As a background of such environmental change, for a white film used as base for the receiving sheet for thermal transfer recording, it is strongly desired, while satisfying whiteness and cushion factor, to improve processing properties such as crease resistance and emboss resistance, and that sensitivity, emboss depth of high concentration region of the printed receiving sheet is small and to be low in cost.
Conventionally, as a base of receiving sheet used for such thermal transfer recording system, various kinds of white polypropylene films have been proposed such as a white film in which an inorganic particle or an immiscible resin such as polyester resin is incorporated in the polypropylene, and the interface between the polypropylene and the inorganic particle or the immiscible resin is peeled off in a drawing process to form voids (i.e., refer to Patent references 1 and 2). However, in the white film in which these proposed void-forming agents, composed of the immiscible resin or inorganic particle, are added, it becomes necessary to add a much amount of the void-forming agent. Accordingly, there are cases in which many protrusions caused by the particles are formed on the film surface to enhance surface roughness, or in the film forming process and in the successive processing steps, the particles fall off to stain the process. Furthermore, there is a case in which, due to a large average dispersed diameter of the immiscible resin or inorganic particle in the film, formed voids become coarse and few, and cushion factor is low and crease resistance and emboss resistance are inferior. And, a receiving sheet for thermal transfer recording in which the white film is used as its base becomes a sheet of a low sensitivity. And, when it is used as a reflection plate, its light reflectance becomes low. In addition, there was a problem that the cost of the obtained film becomes high when, in order to densify voids, particle diameter of the immiscible resin or inorganic particle is made small and/or the particle size distribution is made narrow.
On the other hand, as a method for making voids in the polypropylene film other than the above-mentioned, for example, a method in which, when an undrawn sheet is produced by melt-extruding a polypropylene, β-form crystal (crystal density: 0.922 g/cm3) of which crystal density is low is formed in the undrawn sheet, and by drawing it, the crystal is converted into α-form crystal (crystal density: 0.936 g/cm3) of which crystal density is high to form voids by the difference of crystal density between them, can be mentioned.
Regarding this method, a microporous film obtained by adding β crystal nucleating agent and calcium carbonate to an ethylene-propylene block copolymer and an ethylene containing polypropylene (i.e., refer to Patent reference 3) or, a white film in which a skin layer having heat sealability or printability is laminated on at least one surface of a core layer composed of an orientation acceleration polymer, homopolypropylene, β crystal nucleating agent and an inorganic particle (i.e., refer to Patent reference 4), in addition, a microporous polypropylene film having β-crystal activity and its porosity is increased by an biaxial drawing (i.e., refer to Patent reference 5), or the like are proposed. However, in these proposals, since it is difficult to form receiving layer uniformly, there were problems that sensitivity is low or, surface glossiness is low. Furthermore, there was a problem that emboss depth in the receiving sheet is large when printed.
On the other hand, as other uses of the white film, a reflector or a reflection plate can be mentioned. For example, for a flat type liquid crystal display used for a note type personal computer in which flatness and small size are desired, a side light type, i.e., a backlight which illuminates image surface from a side is applied. The reflector or reflection plate used as a backlight for such liquid crystal image surface, as well as being a thin film, a high reflectance is required, and conventionally, films to which white pigment was added or, films to which inorganic particle or the like was added to include micro voids in their inside have been used. For example, as the light reflector made of polyolefin resin sheet, a light reflector containing polyolefin resin 100 mass parts and fine powdery inorganic-based filler 100 to 300 mass parts and drawn 1.5 to 20 times in areal ratio, and further, of which light reflectance at wavelength of 550 nm is 95% or more, rigidity of said reflector is at least 50 mm, is proposed (i.e., refer to Patent reference 6). However, in thus proposed films, there was a case in which the specific gravity was high and the processability was inferior, and the reflectance was decreased by a diffused reflection or absorption of light due to the resin and the inorganic particle for void formation, and a problem that the resin and the inorganic particle for void formation turned yellow by irradiation such as ultra-violet light, to thereby decrease whiteness of the film. Furthermore, there were problems that the resin and the inorganic particle for void formation fall off and stain the process environment in the film formation process and the light reflection plate production process.    Patent reference 1 JP-H5-78512A    Patent reference 2 JP-2006-181915A    Patent reference 3 JP-H4-309546A    Patent reference 4 WO03/93004 pamphlet    Patent reference 5 WO05/103127 pamphlet    Patent reference 6 JP-H8-262208A